TGD2 proteins of Arabidopsis are proposed to be a substrate binding component of a lipid transfer complex in the inner chloroplast envelope membrane. Loss of function of this protein or other components of this complex may disrupt the endoplasmic reticulum (ER)-pathway of thylakoid lipid biosynthesis. In one embodiment, the present invention contemplates a minimal binding domain capable of specifically binding phosphatidic acid. Alternatively, the minimal binding domain may further comprise accessory binding domains that, in combination, create a complete TGD2 phosphatidic acid binding domain. Consequently, phosphatidic acid may be quantitatively detected from samples as described in the methods herein.